A variety of electronic devices such as portable computers, portable phones, personal digital assistants, and other portable and non-portable electronic devices may utilize one or more DC to DC converters. DC to DC converters generally convert an input DC voltage to a regulated output DC voltage. A DC to DC converter may be utilized to serve a variety of loads within an electronic device. The load served by the DC to DC converter may vary from a relatively light load to a relatively heavy load. The distinction between a light and heavy load may vary based on a particular application, system, and/or user requirement.
Different types of DC to DC converters may be more suitable for serving light or heavy loads. A linear mode voltage regulator may be one type of DC to DC converter that is more suitable to providing power to light loads. The linear mode voltage regulator may monitor changes in output DC voltage and provide a control signal to a transistor to hold the output voltage at the desired value. One type of a linear mode voltage regulator may be a low drop output voltage regulator (LDO) that can provide power to a relatively light load with relatively little voltage drop and with a low noise output. Another type of DC to DC converter may be a switch mode DC to DC converter that holds the output voltage at a desired value by turning at least one transistor of the DC to DC converter ON and OFF. Such a switching type of DC to DC converter may provide a regulated output voltage at a relatively high efficiency when serving a heavy load.
A conventional method that served a load that can be either a light or heavy load under differing conditions provided one DC to DC converter, for serving the light loads, and provided another separate DC to DC converter, e.g., a switching mode DC to DC converter, that served the heavy loads and switched over between each DC to DC converter under certain circumstances. Such a conventional method required two DC to DC converters and additional components and pins to facilitate switching between each adding cost and complexity.
The load current of a modern central processing unit (CPU) is highly dynamic and changes very quickly from low to high and from high to low. A CPU current transient may occur within 1 microsecond, for example, which is less than the typical switching period of conventional voltage regulators.
In the conventional pulse-width modulation (PWM) scheme, the compensation (COMP) output of the error amplifier is typically compared to a fixed ramp signal by a PWM comparator, which generates a PWM signal used to control switching of a DC-DC power regulator. To provide switching noise immunity, a reset-set (R-S) flip-flop is often coupled to the output of the comparator to ensure that there is only one pulse for each switching cycle. A leading-edge modulation scheme is good for the load-adding transient event but not always responsive to a load-releasing transient, while a trailing-edge modulation scheme is good for the load-releasing transient event but not always responsive to a load-adding transient event. Each of these conventional schemes, therefore, inserted clock signal delays under certain load varying situations.
Often, systems required the ability to change the value of the output voltage of the power supply system under the control of an external control system such as a micro-computer. The external control system typically sent a signal to the power supply control system in order to change the value of the output voltage. FIG. 1 illustrates an error amplifier of a typical prior power supply control system 10. System 10 had a PWM control section 17, a power driver stage 18, an error amplifier 14, and a reference voltage 16. Error amplifier 14 included an operational amplifier 15 in addition to a compensation and gain control network that included a first impedance 12 and a second impedance 13. Impedances 12 and 13 typically included resistors and capacitors that were used to provide high frequency stability for changes in the input voltage (Vin) applied to amplifier 14. One problem with these power supply controllers was accuracy. Often, the error amplifier circuit had tracking errors which caused the output voltage to have inaccurate and unstable changes when the control system requested a change in the value of the output voltage. Such inaccuracy and instability detrimentally affected the operation of the control system that used the output voltage of the power control system.
When two DC-DC converters are present on the same input filter ripple current can be an issue, and it becomes important during dynamic loading to respond to transients immediately.
Accordingly, it is desirable to have a system that can reduce input filter ripple current without creating voltage disturbances on the output voltage.